Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Wood is a staple construction material used throughout the world. However, it is prone to degradation from elements including the natural environment, weather events, insects, rot and fire. Accordingly, a range of chemical treatments has been developed to improve the durability and working lifetime of wooden structures.
To treat and prevent infestations, timber is often impregnated with a preservative such as a fungicide or insecticide. The preservative is typically present in a carrier, with the mixture being applied to the surface of the timber, for example by dipping, spraying, brushing or pressure treatment, such that the carrier and preservative are absorbed in to the timber.
The treatment of timber or timber products with preservative compounds involves the introduction of stable chemicals into the cellular structure of the timber. This, in turn, protects the timber from hazards such as fungi, insects and other wood-destroying organisms. Preservative treatments may also include the introduction of chemicals that improve resistance to degradation by fire.
Preservative treatment of wood is often carried out at increased pressure so as to force the liquid preservative solution into the pores of the wood. A vacuum may be applied prior to the introduction of the treatment solution in order to increase penetration. The active chemical agent is usually dissolved in a solvent (e.g. a “carrier”) and the preservative solutions are generally of relatively low viscosity in order to facilitate the penetration of the treatment solution.
Increased penetration of the preservative solution can also be achieved by diffusion, which despite involving less expensive equipment, requires a longer time period and greater levels of stock holding. Diffusion time is also influenced by the initial wood moisture content, especially when dealing with aqueous carriers.
TABLE 1Minimum H3 preservative retention in the penetration zoneWaterborneLight organic solvent preservativesCu +CopperTBTN orPropiconazole +CopperSyntheticCCADDACazoleCreosoteTBTOTebuconazolenaphthenatePyrethroids0.380 Soft: 0.350.2298.000.080Soft: 0.060.100.02 PermethrinHard: 0.390.160Hard: —   .03 Cypermethrin.002 Deltamethrin.0047 Bifenthrin
In Australia, the treatment of timber is covered by the Australian Standard “AS 1604-2010” series. Hazard Class H3 is defined as being for protection against “moderate fungal decay and termite hazard for decking, fascia, cladding, window reveals, and exterior structure timber”. The timber is exposed to the weather or not fully protected. It is clear from the ground and the area is well drained and ventilated. H3 treatment is designed to prevent attack by insects, including termites, and decay. Hazard Class H4 defines “severe decay, borers and termites, fence posts, greenhouses, pergolas (in ground and landscaping timbers”. The timber is in contact with the ground or is continually damp so there is a severe decay hazard. The treatment stops attack by insects, including termites, and severe decay. Retention is measured in mass/mass (% m/m).
“Penetration” is defined under the H3/H4 Standards as: “All preservative-treated wood shall show evidence of distribution of the preservative in the penetration zone in accordance with the following requirements: (a) If the species of timber used is of natural durability class 1 or 2, the preservative shall penetrate all the sapwood. Preservative penetration of the heartwood is not required; (b) If the species of timber used is of natural durability class 3 or 4, the preservative shall penetrate all of the sapwood and, in addition one of the following requirements shall apply; (i) Where the lesser cross-sectional dimension is greater than 35 mm, the penetration shall be not less than 8 mm from any surface. Where the lesser cross-sectional dimension is equal or less than 35 mm, the penetration shall be not less than 5 mm from any surface; (ii) Unpenetrated heartwood shall be permitted, provided that it comprises less than 20% of the cross-section of the piece and does not extend more than halfway through the piece from one surface to the opposite surface and does not exceed half the dimension of the side in the cross-section on which it occurs”.
In order to provide for penetration of the preservative, a carrier must be used. As shown in the Australian Standards, the carriers presently available can be characterised broadly as “water-borne” or “solvent-borne” systems.
A carrier must be capable of providing sufficient penetration of the preservative into the wood, thereby to provide an effective barrier against infestation. Other considerations in the choice of carrier include the desired rate of penetration, cost, environmental, health and safety considerations. A carrier may provide for a “complete penetration” formulation, or for “incomplete penetration”, often referred to as an “envelope treatment” in which one or more preservatives penetrate only partially the cross section of the timber in question.
The preservatives commonly used in timber treatment can be characterised according to the carrier vehicle used to carry preservatives into the timber, and by the active chemicals protecting against the various hazards. The final step in the preservation process is that the solvent must then be removed before the timber is made available for use.
Inorganic boron compounds have been used to protect the sapwood of susceptible hardwoods against lyctid or “powder post” borers. Such treatment consists of soaking freshly-sawn unseasoned timber in solutions of boron salts. The salts diffuse through the timber, thereby treating it, and after such treatment, the timber is allowed to dry. However, boron salts are readily soluble in aqueous solutions and can be leached relatively easily from the wood once treated. This largely restricts boron-treated timber to interior uses such as flooring or joinery, wherein it is protected from the external environment.
Copper, chromium and arsenate (H2AsO4−), “CCA”, is a leach-resistant preservative that has been used to treat solid wood in external applications. CCA impregnates the timber in a water/salts carrier and reacts with the wood cell components so that the active elements are “fixed” into the wood's structure. The arsenic component protects the sapwood from insect attack; the copper and arsenic protect against degradation due to fungi; whilst the chromium chemically locks the elements into the timber, offering a relatively high resistance to leaching. Following such treatment, the timber must be re-dried. This process can decrease the strength of the timber, and invariably adds to the cost. However, due to environmental health and safety issues, CCA is coming under increasing regulation and is thus becoming a less desirable treatment from both commercial and environmental standpoints.
Light Organic Solvent-borne Preservatives (LOSPs) comprise a light organic solvent, typically white spirits, to carry the preservative into the timber. White spirits is a mixture of saturated aliphatic and alicyclic C7-C12 hydrocarbons with a w/w content of about 15-20% aromatic C7-C12 hydrocarbons. The solvent is drawn out in the final stages of treatment, with the preservative remaining within the wood. Such preservatives are typically fungicides, having copper, tin, zinc, azoles and pentachlorophenols (PCPs) as major toxicants. Synthetic pyrethroids such as permethrin may be incorporated within the preservative composition if an insect hazard is also present. One principal advantage of LOSP treatment is that the treated timber does not swell, making such treatment quite suitable for “finished” items such as mouldings and joinery. The majority of LOSPs used in wood treatment also contain insecticides and/or waxes so as to give the surface water repellent properties. However, odour and exposure to VOCs (volatile organic compounds) are significant environmental/occupational health and safety issues. Accordingly, whilst effective, LOSP treatments are becoming increasingly undesirable.
Alkaline Copper Quat (ACQ) contains copper and a quaternary ammonium compound. It is used to protect timber against decay, fungi and insects. ACQ is applied as a water-borne preservative using an external pressure process and is suitable for external timber applications.
Copper azole can be used in water-borne pressure treatment processes. It is a preservative that contains copper, boric acid and tebuconazole. Copper azole has been used in Australia as a replacement for CCA for treatments having external applications.
A range of synthetic pyrethrins and pyrethroids has also been developed for use in treating timber. Many such formulations employ similar chemicals to those used in flea collars for dogs and cats or for fly sprays. Accordingly, such chemicals are already generally well received in the community. However, as preservatives in an aqueous solvent for the treatment of timber, such components are still susceptible to leaching from the timber after treatment. Moreover, swelling of the timber after treatment due to water retention is a significant detriment.
Creosote and PEC (pigment emulsified creosote) are commonly used oil-borne preservatives that are painted onto timber surfaces, but can also be applied in a pressure-based process for better penetration. These compounds have volatile components and hence, a characteristic odour. This makes creosote and PEC only really suitable for use in external or industrial applications.
U.S. Pat. No. 5,846,305 discloses a liquid wood preservative solution including copper metal, liquid amine solvent, a boron compound and a glycol. Glycol has been used as a replacement for water in ancient timber restoration, or for dimensional stability in a technique known as “bulking” for many years. Boron is highly miscible in glycol and thus the movement of a glycol/boron solution into the wood is due to diffusion. Due to the length of time required for adequate diffusion into the wood, this type of preservative requires vacuum pressure application and is thereby unsuitable for the more preferred application methods such as brushing, dipping or spraying.
As the use of water-based carriers has been found to increase the moisture content of the timber, resulting in undesirable swelling of the wood, and necessitating a further drying processes after treatment, many current methods of treating and protecting wood typically rely on using a solvent.
One such composition for the treatment of timber is Tanalith-T (U.S. Pat. No. 7,361,215, to the present Applicant). “Tan-T” uses linseed oil in combination with a high flash solvent carrier to transport a preservative into the wood. This promotes the formation of a well-defined “envelope” of preservative, thereby treating and preventing infestations of termites and other insects. However, with the increasing cost of both vegetable (i.e. “drying”) and mineral oils, there remains a need for the development of other carriers that can provide a protective envelope similar to that of the presently-used vegetable oil/mineral oil carrier formulations, whilst preferably minimising the increase in moisture content of the timber as a result of the treatment, and without need for further drying steps.
WO 02/081159 relates to a method for the protective treatment of wood by thermal treatment at 60-250° C. and additional treatment using an amine and/or amine derivative and/or salt thereof. The treated wood has good resistance even to harmful organisms. The method is carried out without compounds containing heavy metals and wood treated in this way has no impact on the environment. However, it will be appreciated that the initial heat-treatment step is relatively undesirable for cost-energy reasons.
NZ 543124 relates to a process for treating dimensioned wood, dimensioned lumber or dimensioned timber or veneer or particle based products or reconstituted wood products or other cellulosic materials, the said process comprising impregnating the timber or product to its core using a pressure differential or pressure differentials over one or more composition exposure time/s, wherein the composition to which the timber or product is exposed for uptake is a liquid based composition having a biocidal and/or preservative action, and wherein the liquid based composition is at least primarily water based but content does include one surfactant or at least one other liquid (i.e. includes (i) a glycol content or (ii) a glycol content and a surfactant or wetting agent content) and wherein the liquid uptake us less than 80 L/m3.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
Despite the many and varied techniques for the treatment of wood, there remains a need to satisfy the “dry after” requirement for structural treated timber, having less than 15% moisture content, whilst achieving the required penetration of active compounds into the wood.
Additionally, remains a need for a material for treating wood that has relatively low odour, relatively minimal VOC emissions, and relatively less reliance on mineral spirits, whilst at once providing relatively good dimensional stability to the treated wood.
As applied in the context of the present invention, “miscibility” is the property of liquids to mix in all proportions, forming a homogeneous solution. Water and ethanol, for example, are miscible since they mix in all proportions. By contrast, substances are said to be “immiscible” if in some proportion, they do not form a solution, e.g. diethyl ether/water. Moreover, the terms “not miscible” and “immiscible” are taken to be synonymous.
By “operatively support”, as it appears within the claims is intended to mean that the carrier enables the preservative to perform its function within the wood. In other words, the carrier enables the preservative to migrate to its operational extent (i.e., envelope or complete penetration).
Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.
Although the invention will be described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.